I am 39 yr old female, have been working out regularly for nearly a year. Thus far, had successfully used treadmill and elliptical. However, today, after working with a personal trainer for the first time. I got extremely dizzy and everything literally went white and I couldn't hear much of what the trainer said for at least 15 min. I had to sit down immediately and thought I was going to black out. I was quite concerned as this has never happened before. My breathing was fine however, no wheezing, shortness of breath. Any ideas??? I am 5,5- 136lbs. Born with a heart murmur, otherwise no known heart concerns, issues.